[INTERNAL] removed SessionIDObservable dependency

Removed SessionIDObservable dependency from the Feedback classes. Used
session.getID() instead which also will never change during runtime.

Change-Id: I42e0052f817ab4d1db068798a82f8daaacd8f26c
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1139
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>